MEDINA, CATARACT, AND CLINTON CONTACTS 293 



shale. The contact here is sharp and without transition from the sand- 

 stone into the shale. Farther east the transition in the character of the 

 sediments is said to be more gentle and complete, and this condition 

 probably obtains throughout Pennsylvania and Maryland. In the latter 

 state, at Cumberland, a complete and gradual transition can be seen, and 

 it is clear that Coelospira hemispherica gradually appears and finally 

 dominates the Clinton there. 



At Eochester the Clinton consists of four members. From below these 

 are the Sodus shale (24 feet), Wolcott limestone (14), Williamson shale 

 (24), and Irondequoit limestone (18). At Medina, which is 40 miles 

 west of Rochester, the Sodus has thinned to 3 feet, and although there 

 is a basal Clinton shale, sometimes called Sodus, farther west, there is 

 no proof that it holds the time of this member as developed at Eochester. 

 In the Niagara gorge the Clinton, of continuous deposition, is 30 feet 

 thick and represents the time from the Irondequoit to the Wolcott, as no 

 intermediate shale representing the Williamson is developed here or to 

 the westward. Going farther west and then north along the Niagara 

 escarpment, the Clinton limestone gradually pinches out and none is 

 present north of Glenwilliam. In this region the higher Eochester shale 

 pinches out in the same way and none is seen north of Limehouse. For 

 these reasons the Lockport dolomite in these northern sections comes to 

 rest directly on the Cataract, and there is here, therefore, an easily dis- 

 cerned disconformity, indicating a time break of considerable length. 

 [Grabau as censor of this paper thinks there is no break here, only a lat- 

 eral sedimentary change from the Eochester shale to the limestones of the 

 Lockport type. In the same way he explains the broken contacts between 

 the Medina and Clinton. To these views the writer does not assent.:] 

 To the southeast the break becomes less and less long and from Thorold 

 and the Niagara gorge east to Eochester, while the contacts are uneven, 

 with the basal Lockport sediments disturbed and wave-rolled, still there 

 appears to be no break in sedimentation between the Eochester and 

 Lockport. 



On the other hand, the Medina at Eochester is about 60 feet thick, and 

 this depth is maintained to Lockport; but in the Niagara Gorge the 

 thickness is usually given as about 120 feet. A reexamination of this 

 famous Siluric section shows that about 54 feet are referable, on the basis 

 of fossils, to the Cataract, so that the Medina as here restricted has the 

 regulation thickness of about 60 to 65 feet. The contact between them 

 is easily determined, as the Medina is always a sandstone resting on the 

 Cataract shales (see plate 14, figure 1). In tracing the Medina to the 

 northwest, it is seen to pinch out as do the Clinton and Eochester, and is 

 observed for the last time at Dundas, with about 8 feet thickness. The 



XXI — Bull. Geoi,. Soc. Am., Vol. 25, 1913 



